Elevated DNA 5-hydroxymethylation promotes chondrocyte differentiation via TET-mediated epigenetic downregulation of hTERT in human mesenchymal stem cells.

DNA methylation is considered a well-regulated mechanism involved in cellular differentiation, particularly in the differentiation of mesenchymal stem cells into chondrocytes. TET family proteins potentially regulate demethylation by the oxidative conversion of 5-methylcytosine (5-mC) into hydroxymethyl cytosine (5-hmC) and then into formylcytosine (5-fC) and carboxyl cytosine (5-caC) in a series of reactions. However, the precise role of TET proteins and the impact of 5-hmC dynamics during chondrocyte differentiation remain unclear. This study aimed to investigate the potential association of 5-hydroxymethylation with TET family proteins and human telomerase reverse transcriptase (hTERT) gene expression during the differentiation of human mesenchymal stem cells (hMSCs) into chondrocytes. Methodology involves hMSCs differentiation into chondrocytes over 16 days of culture, with successful differentiation confirmed by Alcian Blue staining and chondrogenic marker gene (ACAN and COL2A1). Gene and protein expressions of TET1, TET2, TET3, and hTERT were analyzed through real-time polymerase chain reaction (RT-PCR) and western blot. Global changes in DNA 5-hmC level quantified using dot blot and enzyme-linked immunosorbent assay (ELISA). Locus-specific changes within the hTERT promoter gene were analyzed through locus-specific PCR. We observed a substantial increase in the expression levels of TET family proteins TET1 and TET2, as well as high global 5-hmC levels during chondrocyte differentiation. However, human telomerase reverse transcriptase (hTERT) expression was reduced both at the messenger RNA (mRNA) and protein levels throughout the differentiation process. Furthermore, epigenetic analysis revealed that elevated 5-hmC enrichment at the hTERT promoter region was strongly associated with the downregulation of hTERT expression. Our findings suggest that 5-hmC may serve as a potential epigenetic biomarker and regulatory element in chondrocyte differentiation, offering new insights into skeletal development and cartilage-related disorders.